CADC: add function to fetch CADC file information

Extended the jsa_proc.cadc.fetch module by adding a function
which uses the HTTP HEAD method to retrieve file information
from the CADC data web service.  The common parts of code
between this routine and the fetch_cadc_file function have
been extracted into a private function.
CADC: check for unchanged files in e-transfer
CADC: add force option for etransfer